The Sassanid Empire was weakened after the war of 602-628 against the Byzantine Empire http://bit.ly/2BLg5lV and their defeats at al-Qadisiyyah http://bit.ly/2QwTBvL and Nahavand at the hands of the Caliphate forces effectively ended the empire. Still, representatives of the House of Sasan continued resisting and found allies - the Tang dynasty in China.

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